The invention is based on a magnet system for magnet valves for controlling liquids, in particular for fuel injection valves, of a vehicle.
German patent publication DE 39 21 151 A1 (U.S. patent application Ser. No. 07/487,576 filed Mar. 2, 1990) discloses such a magnet system for a fuel injection valve (see FIG. 3); this magnet system is sketched in FIG. 1, to explain its basic structure.
The known magnet system in FIG. 1 has an electromagnet 1 with an exciter coil 2 which surrounds a cylindrical magnet core 3 forming a magnet pole with a pole face. Coaxially with the magnet core 3, the exciter coil 2 is surrounded by a magnet housing 4, which is magnetically conductively connected on the one hand, via a short-circuit yoke 5, to the face end of the magnet core 3 remote from the pole face and on the other hand to the pole face of the magnet core 3, via an annular land 6 with a magnetic constriction 7. Coaxially with the magnet core 3, a thin, disk-shaped permanent magnet 8, which is covered by an annular pole plate 9, is seated on the annular land 6. Opposite the magnet pole formed by the magnet core 3 is an armature 10, which extends part way over the pole plate 9 and toward the pole face forms a working air gap 11. The disposition of the permanent magnet 8 and the circulation of the exciter coil 2 are selected such that the magnetic flux of the permanent magnet 8 and the magnetic flux of the electromagnet 1 are opposed to one another in the working air gap 11. The armature 10, firmly connected to the valve member of the magnet valve, is embodied as free-floating. When the electromagnet 1 is unexcited, the armature 10 is kept attracted to the magnet core 3 by the permanent magnet 8, counter to the hydraulic pressure exerted in the valve chamber on the valve member. Upon excitation of the electromagnet 1, the magnetic flux of the permanent magnet 8 in the working air gap 11 is weakened, so that its retention force acting upon the armature 10 decreases to such a point that the armature 10 lifts from the magnet core 3 because of the hydraulic counter force and as a result opens the valve.
The magnetic flux generated by the exciter coil 2 is designated by the symbol .phi..sub.E, and that generated by the permanent magnet 8 is represented in FIG. 1 by .phi..sub.P. It can be seen clearly that the magnetic flux .phi..sub.E develops, via the armature 10, working air gap 11, magnet core 3, short-circuit yoke 5, magnet housing 4, permanent magnet 8 and pole plate 9, into two magnet circuits that are symmetrical with the axis of the magnet system. Since the permanent magnet 8 has a permeability like that of air, it generates a relatively high magnetic resistance in the magnet circuit of the electromagnet 1, and this has to be compensated for with an increased triggering output of the exciter coil. To reduce the magnetic resistance, the cross-sectional area of the permanent magnet 8 is therefore made relatively large, while the slight thickness that as a result is possible for the permanent magnet 8 results from the necessary magnetic voltage and the coercive field intensity, which is as large as possible. Because of its larger area, the eddy current losses in the permanent magnet 8 are larger as well. Thus, large permanent magnets 8 are subject to considerable danger of breakage when they are machined, which considerably increases their manufacturing costs. To reduce the eddy current losses, the permanent magnet 8 is manufactured from cobalt-samarium, which is of relatively low resistance but on the other hand is quite brittle, so that the danger of breakage in magnet machining is increased still further. As already mentioned, the free-floating armature 10 is raised from the magnet pole exclusively by the hydraulic counterpressure exerted on the valve member of the magnet valve. The hydraulic counterpressure decreases sharply during the opening phase of the magnet valve and sometimes even becomes negative. A magnetic force of reversing polarity would therefore be desirable to reliably keep the valve open. Even upon reversal of the magnetic flux in the armature 10, this is impossible, however, since the magnetic force is proportional to (.phi..sub.P -.phi..sub.E).sup.2, or in other words is proportional to the square of the difference in magnetic flux.